Holding weeds in check is a rather big problem in farming, usually solved with aggressive chemicals, or the combination of even more aggressive chemicals with crop-plants genetically altered to survive those.

The genetic modifications needed for crops to become resistant to plague, chemicals, etc.
are rather extensive, and often engender fear about those modifications escaping to weeds, or having unknown influence on other organisms in general.

My proposal needs genetic modification of the crop-plants, too, but on a far more bening level, that is already well-studied, and generally under control : green fluorescent protein, GFP.

Robots today are very fast, but to have them help in farming they would need to be able to make a difference between crop and weed, which they currently can't - the image recognition techniques are just not that good, and in many stages of plant developement it is simply impossible, even for humans.

But if the crop plants would have a GFP (not necessarily green, different colors could code for different crops), a robot would be able to weed out any other plant fast and efficient. The weeds could either help power the robot, or could be mulched.

No chemicals would be needed against weeds (as weeds are plants too, the chemicals going after the weeds always were tricky, as they had to exclude the crop from the slaughter), and as even the fastest-growing weed takes some time, the robot would not need to be supersonic.

It just occurred to me, given the precision of modern
planters, it would probably be possible to produce a robot
that pulls or cuts anything that isn't growing within the
probable growth radius of each seed. This won't help if you
end up with a weed within a half inch to an inch of where the
seed was planted, but should deal with the rest.